Comment 2 for bug 568895

well indeed the behaviour you're depicted will occur but according to me, in the current situation, there isn't any other choice.

I think that the ones that want to invoice the services more accurately should install the module project_mrp and invoice based on timesheet entr, or you can also set you invoicing policy as manual and invoice line by line, using the menu Sales\Invoicing\Lines to Invoice. Both seems a better solution.